Two complementary new surveys of the x-ray background (XRB), the WHDF and the 10 X 10 ks, are presented. 140 serendipitous x-ray hard and soft sources (S(_2)-10 keV 3. 10(^15); S0.5-2 keV 4 . 10(^16) ergcm(^2)s(^1) have been identified and characterised by conducting concurrent optical and x-ray observations. A principal aim of this work has been to establish whether x-ray luminous narrow-emission line galaxies (NELGs) are the sources that are the major contributors to the hard XRB, along with finding an explanation for their emission mechanisms. We build a case for a hidden AGN as the most likely explanation for such emission and, while NELGs are indeed found to be major contributors to the hard XRB, they are so as the nearby representatives of a major class of obscured AGN, most of which are too faint for probing with current spectroscopic facilities and appear either as "normal" galaxies or as blank fields in optical observations. In particular, we find no evidence of significant contribution from starbursts to the XRB intensity. We also explore the high-redshift population of luminous absorbed AGN and report on the discovery of a type QSO candidate at z = 2.12. But the number of such sources observed is found to be significantly below the predictions from obscured AGN models of the XRB and, inspired by the discovery of several broad-line quasars amongst the hardest sources in the WHDF and also in other surveys, we suggest that x-ray luminous absorbed AGN show optical broad lines more often than not. This affects the relationship between gas and dust in AGN and has direct consequences for the basic unification schemes for AGN. In a parallel program, not only to study how the stellar content of the present universe was assembled over time but also to understand the photometric properties of the galaxies that host an AGN, we perform detailed analysis of the evolutionary properties of early-type galaxies. We find that a significant fraction of colour-selected elliptical and lenticular galaxies in the direction of the WHDF show colours that are too blue to be consistent with the predictions of a simple mono-littiic collapse at high-redshift and passive evolution thereafter. Their large scatter in photometric observables seems to imply divergent histories and indicate that the early-type populations are rather heterogeneous. In particular, significant low-redshift star formation is deduced from the large scatter in their colour-magnitude relation and from the presence of [OII]ƛ3727 emission lines.